The invention relates to the field of antenna communication systems. More particularly, the present invention relates to phase array antenna systems communicating dual frequency signals generating intermodulation grating lobes.
Communication systems use antennas for transmitting and receiving communication signals. The communication systems can use a variety of antenna systems having transmitter and receiver antennas for defining antenna gain patterns with maximas for directional transmitting and receiving the communication signals. One type of antenna system is the active transmit phased arrays having multiple directional antenna elements using beam steering. Typically, the phased array antenna has a plurality of individual antenna elements lying in plane. Each antenna element broadcasts one or more steered communication signals eliminating the need for multiple apertures. Each array element has a respective phase offset for each signal for steering the respective antenna beams in a desired direction toward communication receivers. The transmission of the multiple communication signals create unwanted intermodulation products in power amplifiers that produce gain patterns appearing as unwanted signal at intermodulation frequencies in secondary intermodulation main beams and grating lobes in the antenna gain pattern. Transmitter power amplifier linearizers and power back-off methods are used to reduce signal distortion. While solid state power amplifier linearizers and power back-off techniques can lower the levels of the unwanted intermodulation products, such techniques lower the array efficiency. It is desirable to control the phased array elements with grating lobe suppression for reduced signal distortion during signal transmission that may use saturated power amplifiers and linearization methods.
Active phased arrays have solid state power amplifiers at each array element. These solid state power amplifiers are nonlinear devices that produce the unwanted intermodulation products when multiple signals are introduced. The intermodulation frequencies are spaced according to the difference between the frequencies. For example, when two transmit carrier frequencies f1 and f2 are used for broadcasting signals with two primary main beams creating unwanted intermodulation frequencies at 2f1xe2x88x92f2 and 2f2xe2x88x92f1. The phased array produces antenna patterns at the intermodulation frequencies. The secondary intermodulation main beams of the intermodulation product patterns are steered according to the difference in the pointing angles of the primary main beams. Therefore, the phased array antenna field of view contains the two primary main beams and may contain intermodulation grating lobe beams depending on the difference in pointing angles of the two primary main beam patterns. When the primary main beams are closely spaced, then the secondary intermodulation main beams and intermodulation grating lobe beam may appear within the filed of view of the phase array antenna. When the primary main beams are widely spaced, then a special condition occurs where the secondary intermodulation main beams advantageously appear outside the field of view and the intermodulation grating lobe beams disadvantageously appear within the field of view. When the intermodulation grating lobe beam are in the field of view, then the intermodulation grating lobe beams are unwanted interference generated at the intermodulation frequencies. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide a method for reducing interference generated at intermodulation frequencies.
Another object of the invention is to provide a method for suppressing intermodulation grating lobe beams at the intermodulation frequencies.
Another object of the invention is to provide a method for suppressing intermodulation grating lobes in phased array antenna systems.
Still another object of the invention is to provide a method for reducing transmitted interference signals generated at intermodulation frequencies.
Yet another object of the invention is to provide a method for suppressing intermodulation grating lobes in phased array antenna systems by regular spacing of phase array antenna elements.
The present invention is directed to a method for reducing the amplitude level of intermodulation grating lobes in the field of view of a phased array antenna system by regular spacing of subarrays each having a plurality of phased array antenna elements and by interposing predetermined regular spacing between the subarray antenna elements within each subarray. The secondary intermodulation main beams are disposed off of the field of view, but the remaining intermodulation grating lobes beams still appear within the field of view, but at suppressed amplitudes levels. The regular spacing of the subarrays and the regular spacing of the phased array antenna elements within each subarray reduces, that is, suppresses the intermodulation grating lobe beams that may appear in the field of view. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.